Ghost Sharks

It isn’t easy to find a ghost shark. Most hide in the eternal darkness hundreds of meters below the thin slice of ocean that we generally visit. But, along the shorelines of Puget Sound and British Columbia, one species regularly materializes out of the gloom to the delight of surprised scuba divers.

Spotted Ratfish (Hydrolagus colliei) are not really reef dwellers. They forage for food in the sand and mud so the best way to get a close look at one is to swim out onto the sand flats at night where they can be seen flapping around, mouth slightly agape, sifting through the silt in search of clams and small crustaceans. When caught in the beam from a diver’s flash light, their ghostly silhouettes shimmer metallically, and their eyes glow a frosty emerald green.

Spotted Ratfish and other ghost sharks (subclass holocephali) only loosely resemble true sharks (subclass elasmobranchi). They have the same cartilaginous skeletons but unlike sharks, their upper jaws are fused to their brain cases. They also have a flap of skin called an operculum, which covers their gill arches. Another immediately obvious difference is in the way that they swim. Ratfish achieve forward momentum by flapping their broad pectoral fins while their skinny, whip-like tails undulate from side to side in a vaguely shark-like way.

If you think that their appearance is odd, wait until you hear about ratfish sex.

Male ratfish have claspers extending from their anal fins just like sharks. They use them to transfer sperm to the female during copulation. Male sharks usually bite the female’s flank or pectoral fin to maintain position while they are coupled, but ratfish have a more unusual strategy. They have another clasper in the middle of their foreheads – no, I’m not making this up. This extra appendage has tiny barbs on its underside and can be used like a Velcro covered arm to stop the male from falling away before he’s finished. Maybe dating would be much simpler if we all displayed our sexual intentions in the middle of our foreheads.

Once the female ratfish is impregnated, two embryonic offspring begin to grow inside leathery egg cases that the female carries around until she is ready to deposit them onto the reef. These cases resemble the rough shape of the adult with a large oval for the body and a tapering, feathery extension for the tail. Sometimes, discarded egg cases can be seen lodged between rocks or rolling around on the seafloor.

Dodging sixgills, dogfish, and their other aggressive relatives, the newly hatched ratfish immediately disappear into the depths. Where exactly they go, and the perils they face in their lightless world, is anyone’s guess. Most young ratfish probably end up in the bellies of ferocious looking deep sea fish. Their only defense against this is a tall venomous spine on the leading edge of their first dorsal fin. This painful weapon can be held high to inflict a nasty puncture wound on any mouth snapping shut around it.

Ghost sharks have been lurking in the depths for the last 340 million years. Initially they flourished alongside the true sharks, and the fossil record contains evidence of many bizarre looking species, some of which may have grown up to 3 meters in length but perhaps unable to compete for space with their fiercer cousins, they eventually diminished both in size and numbers. Today there are only three families left, containing around 34 species and most of these are condemned to a life in the inhospitable depths.

Ghost sharks may be a rare site for divers, but if you drag yourself away from the reef long enough to look, you could be rewarded by an encounter with one of nature’s oddest creatures; a remnant from an evolutionary branch of fishes almost eradicated from today’s shark dominated oceans.

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